1. Field of the Invention
This invention relates to the photoelectro-chemical etching system of compound semiconductor, which is employed to achieve an etch at high rate and enable the homogeneous surface of the semiconductor after etching, by modulating appropriately both the waveform period of laser beam used as an optical source and the waveform of voltage applied to semiconductor material imbedded in etchant solution. It especially relates to system of this kind which is applicable in forming the diffraction lattice in photoelectro-chemical laser holography in line with the manufacture of optical elements such as DFB semiconductor lasers and Bragg reflector semiconductor lasers, and extensively to a method used in treating the surface of compound semiconductor material in an efficient manner.
2. Description of the Prior Art
In general, the photoelectro-chemical etching method using lasers, which is a novel technology that can selectively etch the desired parts without using a mask, is extensively used for manufacturing such optical elements as LED(Light Emitting Diode), long wavelength mesh p-i-n photodetector, and millimeter wave electro-oscillator. The following is a summary and some disadvantages as to when the diffraction lattice in laser holography is photoelectro-chemically formed on the surface of the compound semiconductor. In the prior art, a sufficiently large reverse voltage during the etching process must be applied so as to stimulate the oxidation reaction of semiconductor material deposited in etchant solutions. As the etching proceeds, many holes under the reverse-bias condition are induced on the surface of n-type compound semiconductor and this is further facilitated by the illumination of lasers. In a photo-chemical reaction between lasers and etching solutions, the semiconductor material is positively ionized so that some by-products generated from the process are deposited into the etchant solutions. In this case, a majority of by-products are present on the surface of the material in hydroxide, or form some layers in liquid by bonding with negative ions of etchant solutions near the surface of the semiconductor. If the layers of by-product formed during the etching process are above certain thickness, they act as optical waveguide and with the illumination of laser, and the incident light is absorbed into the layers which gives rise to oscillation and reflection. This degrades the intensity of lasers and the undesired parts are hence etched. Also, as the refractive index in layers of by-product differs from that of the etchant solution, the incident light will cause the change of incidence angle, thus affecting the spacing when the diffraction lattice using a holography is to be manufactured. After completion of the etching process, the surface of the semiconductor materials becomes toughened due to the etching of undesired parts, which leads to reduction of the efficiency of the manufactured diffraction lattice.
It is a principle object of the present invention, therefore, to provide the photoelectro-chemical etching system of compound semiconductor, visualizing more efficient etching process than heretobefore available, the above-said objective should be construed as only one of many possible through the utilization of a few of the more practical and important features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objectives and a fuller understanding of the invention may be had by referring to both the summary of the invention and the detailed description, below, which describe the preferred embodiments and describe the scope of the invention defined by the claims, whose summary and description should be considered in conjunction with the accompanying drawings.